What is the smallest blood vessel in the body? | Reference.com What is the smallest blood vessel in the body? A: Quick Answer The smallest blood vessels in the human body are capillaries, which connect arteries and veins. Capillaries can be as small as 5 micrometers wide. The average adult body has about 10 billion capillaries. Full Answer Capillaries are part of the circulation system in the human body. They help move oxygen, water, carbon dioxide and other nutrients throughout the body. Arteries transfer blood out of the heart. Arteries narrow into capillaries, then widen into venules and veins. The veins transport blood back into the heart. Capillaries work as a group and not individually. During exercise, capillaries assist the lungs with meeting the need for increased blood flow. Blood Vessels | The Franklin Institute The Franklin Institute Blood Vessels Blood vessels may be tiny but they cover a lot of ground. The smallest blood vessels measure only five micrometers. To give you some perspective, a strand of human hair measures about 17 micrometers.

But if you took all the blood vessels out of an average child and laid them out in one line, the line would stretch over 60,000 miles. An adult’s would be closer to 100,000 miles long. There are three kinds of blood vessels: arteries, veins, and capillaries. Each of these plays a very specific role in the circulation process. Arteries carry oxygenated blood away from the heart. They’re tough on the outside but they contain a smooth interior layer of epithelial cells that allows blood to flow easily. Arteries also contain a strong, muscular middle layer that helps pump blood through the body. Capillaries connect the arteries to veins. The arteries deliver the oxygen-rich blood to the capillaries, where the actual exchange of oxygen and carbon dioxide occurs. The capillaries then deliver the waste-rich blood to the veins for transport back to the lungs and heart. Veins carry the blood back to the heart. They’re similar to arteries but not as strong or as thick. Unlike arteries, veins contain valves that ensure blood flows in only one direction.

(Arteries don’t require valves because pressure from the heart is so strong that blood is only able to flow in one direction.) Valves also help blood travel back to the heart against the force of gravity. SEER Training:Classification & Structure of Blood Vessels Classification & Structure of Blood Vessels Updates Classification & Structure of Blood Vessels Blood vessels are the channels or conduits through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. One system, the pulmonary vessels, transports blood from the right ventricle to the lungs and back to the left atrium. The other system, the systemic vessels, carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins. Arteries Arteries carry blood away from the heart. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues.

Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries. About 10 percent of the total blood volume is in the systemic arterial system at any given time. The wall of an artery consists of three layers. The innermost layer, the tunica intima (also called tunica interna), is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel.

Capillaries Capillaries, the smallest and most numerous of the blood vessels, form the connection between the vessels that carry blood away from the heart (arteries) and the vessels that return blood to the heart (veins). The primary function of capillaries is the exchange of materials between the blood and tissue cells. Capillary distribution varies with the metabolic activity of body tissues. Tissues such as skeletal muscle, liver, and kidney have extensive capillary networks because they are metabolically active and require an abundant supply of oxygen and nutrients. Other tissues, such as connective tissue, have a less abundant supply of capillaries. The epidermis of the skin and the lens and cornea of the eye completely lack a capillary network. About 5 percent of the total blood volume is in the systemic capillaries at any given time. Another 10 percent is in the lungs. Smooth muscle cells in the arterioles where they branch to form capillaries regulate blood flow from the arterioles into the capillaries. Veins Veins carry blood toward the heart. After blood passes through the capillaries, it enters the smallest veins, called venules.

From the venules, it flows into progressively larger and larger veins until it reaches the heart. In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs. Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells. The walls of veins have the same three layers as the arteries. Although all the layers are present, there is less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries, which is related to the fact that blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. Medium and large veins have venous valves, similar to the semilunar valves associated with the heart, that help keep the blood flowing toward the heart.

Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity. Blood Vessels - Heart and Blood Vessel Disorders - Merck Manuals Consumer Version Heart and Blood Vessel Disorders Find information on medical topics, symptoms, drugs, procedures, news and more, written in everyday language. * This is the Consumer Version. * Effects of Aging on the Heart and Blood Vessels The blood vessels consist of arteries, arterioles, capillaries, venules, and veins. All blood is carried in these vessels. The arteries, which are strong, flexible, and resilient, carry blood away from the heart and bear the highest blood pressures. Because arteries are elastic, they narrow (recoil) passively when the heart is relaxing between beats and thus help maintain blood pressure. The arteries branch into smaller and smaller vessels, eventually becoming very small vessels called arterioles. Arteries and arterioles have muscular walls that can adjust their diameter to increase or decrease blood flow to a particular part of the body.

Capillaries are tiny, extremely thin-walled vessels that act as a bridge between arteries (which carry blood away from the heart) and veins (which carry blood back to the heart). The thin walls of the capillaries allow oxygen and nutrients to pass from the blood into tissues and allow waste products to pass from tissues into the blood. Blood flows from the capillaries into very small veins called venules, then into the veins that lead back to the heart. Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen (dilate) as the amount of fluid in them increases. Some veins, particularly veins in the legs, have valves in them, to prevent blood from flowing backward. When these valves leak, the backflow of blood can cause the veins to stretch and become elongated and convoluted (tortuous). Stretched, tortuous veins near the body's surface are called varicose veins (see Varicose Veins ). Blood Vessels: Circulating the Blood Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles. Arterioles connect with even smaller blood vessels called capillaries.

Through the thin walls of the capillaries, oxygen and nutrients pass from blood into tissues, and waste products pass from tissues into blood. From the capillaries, blood passes into venules, then into veins to return to the heart. Arteries and arterioles have relatively thick muscular walls because blood pressure in them is high and because they must adjust their diameter to maintain blood pressure and to control blood flow. Veins and venules have much thinner, less muscular walls than arteries and arterioles, largely because the pressure in veins and venules is much lower. Veins may dilate to accommodate increased blood volume. If a blood vessel breaks, tears, or is cut, blood leaks out, causing bleeding. Blood may flow out of the body, as external bleeding, or it may flow into the spaces around organs or directly into organs, as internal bleeding.

Resources In This Article Blood Vessels and the Circulatory System: Arteries, Veins and Capillaries Comparing Veins, Arteries and Capillaries for Students written by: Rose Kivi • edited by: Amanda Grove • updated: 1/5/2012 The average adult body contains between 9 and 12 pints of blood. Blood contains oxygen and nutrients the body needs to survive. The arteries, veins and capillaries are types of blood vessels that transport the vital blood throughout the body. Learn the differences between the types of blood vessels. slide 1 of 7 The Transportation Highway Blood vessels are hollow, round tubes that act as a transportation highway for the body. Blood vessels are like little pipes that connect to the heart. The heart is like a pump that forces the blood through the blood vessels. Together, the heart and the blood vessels, make up the circulatory system. slide 2 of 7 The Circulatory System Veins, arteries and capillaries are the three types of blood vessels in the circulatory system. The blood vessels carry blood, which contains antibodies, nutrients, oxygen, electrolytes and hormones, throughout the body to be used by the organs.

Oxygen rich blood is pumped out of the left ventricle of the heart into the aorta, which is the largest artery in the body. The oxygen rich blood travels through the aorta into smaller arteries. The capillaries sit between the arteries and the veins, connecting the two together and acting as an exchange stop. They remove oxygen and nutrients from the blood in the arteries and distribute them to the tissues in the body. They also place cellular waste, including carbon dioxide, into the veins. Blood, with most of the oxygen removed, travels through the veins and makes a stop by the lungs to release the carbon dioxide and re-oxygenate, before heading back to the right side of the heart, where the process starts again. The entire process of the blood traveling from the heart, through the arteries, back through the veins and back to the heart, takes approximately one to two minutes. slide 3 of 7 Artery Facts Arteries carry oxygenated blood away from the heart. There are three main classifications of arteries: large or elastic, medium or muscular and small or arterioles. The blood in arteries is bright red in color because it contains oxygen. Arteries have to be strong to withstand pressure from the beating heart.

To withstand the pressure, the arteries contain elastic tissue and muscle that allow them to expand and constrict. The largest arteries are closest to the heart and are about the diameter of your thumb. The aorta, which connects to the left ventricle of the heart, is the largest artery in the body. The smallest arteries, called arterioles, are less than 0.5 mm in diameter, meaning they are so tiny that they are barely visible to the human eye. You can feel some arteries by touching your skin. Feeling the arteries is how you check your pulse. The arteries you can feel by touching your skin are the abdominal aorta, brachial, common carotid, dorsalis pedis, femoral, posterior tibial and the radial artery. slide 4 of 7 Veins Veins carry blood from throughout the body back to the heart. The blood in the veins is dark red in color because it contains very little oxygen. The walls of veins are not as strong as the walls of the arteries because they farther away from the heart, meaning they are subject to less pressure from the heart.

Because veins have less muscle and elastic tissue than arteries, they do not always keep their tube-like shape and can appear flattened in some areas. Veins have to push blood back to the heart, against gravity at times. Some veins have valves that prevent the blood from flowing backwards away from the heart. This ensures that the same amount of blood that left the heart through the arteries goes back to the heart through the veins. slide 5 of 7 The Cardiovascular System (Blood Vessels) The Cardiovascular System (Blood Vessels) I. Blood Vessels: Overview Blood vessels of the body form a closed delivery system that begins and ends at the heart. Three major types of blood vessels: arteries, veins, and capillaries 60,000 miles of blood vessels in body A.

Structure of Blood Vessel Walls Diagram Diagram Diagram Arteries and Veins are composed of three distinct layers called tunics which surround the blood-containing space (lumen) Tunica intima (interna) - contains epithelium (simple squamal) Tunica media - circularly arranged smooth muscle cells and elastic fibers (in arteries); tunica media has myogenic activity (vasoconstriction and vasodilation) Tunica adventitia (externa) - loosely woven collagen fibers and in large blood vessels contain tiny blood vessel, vasa vasorum, that supplies blood to nourish tunica adventitia B. Arterial System - arteries are vessels that transport blood away from the heart and consists of three groups: Elastic (conducting) arteries - large, thick-walled arteries near to the heart (ex: aorta). Their large diameter lumens allows elastic arteries to serve as low-resistance conduits for conducting blood from heart to medium-sized arteries. Elastic arteries expand and recoil passively to accommodate changes in blood volume, keeping blood under continuous pressure. Blood flows continuously rather than directed by pulsatile rhythm of the heartbeat.

Muscular (distributing) arteries - arteries composed proportionately more smooth muscle and less elastic tissue and delivers blood to specific body organs. Arterioles - smallest of arterial vessels; smallest arterioles have single layer of smooth muscle cells spiralling around endothelial lining Arterial Pulse - alternating expansion & recoil of elastic arteries during each cardiac cycle creates a pressure wave = pulse Pulse point (also referred to as compression/pressure points) arteries: Temporal, femoral, facial, popliteal, common carotid, posterior tibial, brachial, dorsalis pedis, and radial C. Venous System - blood is carried from the capillary beds toward the heart by veins Venules - are formed when capillaries unit Veins - have thin tunics and larger lumens than arteries, therefore can accommodate larger blood volumes. Due to the fact that 65% of the blood supply is always in veins, veins are referred to as capacitance vessels or blood reservoirs.

Blood Pressure is low in veins - adaptations to help return blood the heart at the same rate it was pumped into circulation: Large diameter lumen Presence of valves (in limbs) that prevent blood from flowing backward (therefore unidirectional flow) Valves found in veins of limbs not ventral cavity because upward flow of blood is opposed by gravity. D. Capillaries - smallest blood vessels with thin walls consisting of only a tunica intima; Capillary Types : Continuous capillaries - abundant in skin and muscle and are continuous because their endothelial cells provide an uninterrupted lining. Adjacent cells are joined laterally by tight junctions but usually incomplete and leave gaps of unjoined membranes called intercellular clefts. Fenestrated capillaries - endothelial cells have many oval pores (fenestrations) that are covered by a membrane and therefore has greater permeability to fluids and small solutes.

Sinusoids - are capillaries connecting arterioles and venuoles in liver and have large irregular shaped lumens, are fenestrated, and have fewer tight junctions but larger intercellular clefts Capillary Beds - interweaving networks of capillaries consisting of two types of vessels: A vascular shunt (interarteriole-thoroughfare channel) which is a short vessel that directly connects the arteriole and venule at opposite ends of the bed True capillaries Note: The terminal arteriole feeding the bed leads into a metarteriol (a vessel structurally intermediate between an artery and a capillary) , which is directly continuous with the thoroughfare channel (intermediate between a venule and a capillary). The thoroughfare channel, in turn, joins the postcapillary venule that drains the bed. E.

Capillary Exchange Vesicular Transport (exocytosis and endocytosis) Bulk Flow Bulk Flow --> regulation of the relative volumes of blood and interstitial fluid Filtration and reabsorption of fluids due to pressure ( ) differences Balance of pressures = net filtration pressure (NFP) Starling's Law of the Capillaries - movement of fluid between plasma and the interstitial fluid is in a state of near equilibrium at the arterial and venous ends of a capillary NFP = (Blood Hydrostatic Pressure + Interstitial Fluid Osmotic Pressure) - (Blood Colloid Osmotic Pressure + Interstitial Fluid Hydrostatic Pressure) blood hydrostatic pressure = pressure of blood acting against the capillary walls forcing fluids out of capillaries interstitial fluid hydrostatic pressure = pressure acting outside the capillaries pushing fluid in blood colloid osmotic pressure = pressure as a result of moving fluid into cappillaries due to attraction to non diffusable solutes interstitial fluid osmotic pressure = pressure of blood acting against the cappillary walls forcing fluids into capillaries Edema - filtration exceeds reabsorption resulting in increased interstitial fluid volume; causes include: Increased blood hydrostatic pressure in capillaries Decreased concentration of plasma proteins

 Increased permeability of capillaries 2.

Output- vagus (X) nerve - decreases heart rate Cardiac accelerator nerves - increase heart rate and contractility Vasomotor nerves - vasodilation and vasoconstriction Hormonal Regulation Renin-angiotensin mechanism: renin released by kidneys leads to the formation of angiotensin II angiotensin II - causes vasoconstriction of arterioles and increases blood pressure aldosterone - angiotenisn II stimulates release of aldosterone which promotes sodium/water reabsorption in kidney; therefore aldosterone increases blood volume and pressure Epinephrine/norepinephrine - increases heart rate and contraction thereby increasing cardia output and BP Antidiuretic hormone, ADH - released by the hypothalamus increases water retention in the kidney; increases BP causes intense vasoconstriction of arterioles and increases BP Atrial natriuretic peptide, ANP - protein produced by the heart and causes vasodilation of arterioles and promotes loss of salt and water in urine; decreases BP PTH (vasodilator) and Calcitrol (vasoconstrictor) - regulates calcium and phosphate ions, in turn regulates smooth muscle contraction other chemical but non hormonal factors: Nitric oxide (vasodil

ator) What causes capillaries to dilate?

| Reference.com What causes capillaries to dilate? A: Quick Answer Dilated capillaries are caused by a variety of factors, including radiation, sun damage, pregnancy, unbalanced estrogen levels and rosacea, according to Care Fair. They may also be caused by prescription medications or autoimmune diseases. Lifestyle factors like heavy drinking, smoking, or harsh scrubbing of the face can also cause dilated capillaries. Occasionally, dilated capillaries may appear in skin without medical problems due to heredity or the natural aging process, according to Dr. David Green. Full Answer Dilated capillaries are also known as broken capillaries, broken blood vessels or broken veins. The term "broken" is misleading, according to Dr. Patricia Wong, medical and aesthetic dermatologist in Palo Alto, Calif. The capillaries are not actually broken, but inflamed. They are tiny veins which cluster together near the surface of the skin, causing a red, pink or purple bruise-like area. The skin takes on a chronically flushed look, especially during periods of heavy exertion or hot weather.

Dilated capillaries most often appear on the face, especially around the nose and mouth, but may also appear on the legs. While dilated capillaries are generally considered harmless, they do pose a cosmetic concern for those who suffer from them. According to Care Fair, many treatment options exist to remove dilated capillaries or diminish their appearance; these options include electrolysis, laser surgery and cosmetic surgery. Any treatments must be performed by a dermatologist or highly trained esthetician. Arteries, Veins, and Capillaries About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Arteries, Veins, and Capillaries Blood vessels include arteries, capillaries, and veins which are responsible for transporting blood throughout the body.

Learning Objective Explain the structure of arteries, veins, and capillaries and how blood flows through the body Key Points Arteries carry blood away from the heart; the main artery is the aorta . Smaller arteries called arterioles diverge into capillary beds, which contain 10-100 capillaries that branch among the cells and tissues of the body. Capillaries carry blood away from the body and exchange nutrients , waste, and oxygen with tissues at the cellular level. Veins are blood vessels that bring blood back to the heart and drain blood from organs and limbs. Capillaries have one layer of cells (the endothelial tunic or tunica intima) where diffusion and exchange of materials takes place. Veins and arteries have two more tunics that surround the endothelium: the middle tunica media is composed of smooth muscle that regulates blood flow, while the outer tunica externa is connective tissue that supports blood vessels. Terms Full Text Arteries, Veins, and Capillaries The blood from the heart is carried through the body by a complex network of blood vessels . Arteries take blood away from the heart.

The main artery is the aorta that branches into other major arteries, which take blood to different limbs and organs. These major arteries include the carotid artery, which takes blood to the brain; the brachial arteries, which take blood to the arms; and the thoracic artery, which takes blood to the thorax and then into the hepatic, renal , and gastric arteries for the liver, kidneys, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then into smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body. The blood from the heart is carried through the body by a complex network of blood vessels. This diagram illustrates the major human arteries and veins of the human body. Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells in single-file lines and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level.

Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins, which connect to major veins that take blood high in carbon dioxide back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system. The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels . The inner, tunica intima is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. This tunic is continuous with the endocardium of the heart. Unlike veins and arteries, capillaries have only one tunic; this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction , narrowing of the blood vessels, and vasodilation , widening of the blood vessels; this is important in the overall regulation of blood pressure.

Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the endothelium tunic (tunica intima). Veins and arteries both have two further tunics that surround the endothelium: the middle, tunica media is composed of smooth muscle, while the outer tunica externa is connective tissue (collagen and elastic fibers). The elastic, connective tissue stretches and supports the blood vessels, while the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly-pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different from arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart.

Cardiovascular System - Human Veins, Arteries, Heart Cardiovascular System Home > Cardiovascular System Cardiovascular System The cardiovascular system consists of the heart, blood vessels, and the approximately 5 liters of blood that the blood vessels transport. Responsible for transporting oxygen, nutrients, hormones, and cellular waste products throughout the body, the cardiovascular system is powered by the body’s hardest-working organ — the heart, which is only about the size of a closed fist. Even at rest, the average heart easily pumps over 5 liters of blood throughout the body every minute.... Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image Cardiovascular System Anatomy The Heart The heart is a muscular pumping organ located medial to the lungs along the body’s midline in the thoracic region. The bottom tip of the heart, known as its apex, is turned to the left, so that about 2/3 of the heart is located on the body’s left side with the other 1/3 on right.

The top of the heart, known as the heart’s base, connects to the great blood vessels of the body: the aorta , vena cava, pulmonary trunk, and pulmonary veins. Circulatory Loops There are 2 primary circulatory loops in the human body: the pulmonary circulation loop and the systemic circulation loop. Pulmonary circulation transports deoxygenated blood from the right side of the heart to the lungs , where the blood picks up oxygen and returns to the left side of the heart. The pumping chambers of the heart that support the pulmonary circulation loop are the right atrium and right ventricle. Systemic circulation carries highly oxygenated blood from the left side of the heart to all of the tissues of the body (with the exception of the heart and lungs). Systemic circulation removes wastes from body tissues and returns deoxygenated blood to the right side of the heart. The left atrium and left ventricle of the heart are the pumping chambers for the systemic circulation loop. Blood Vessels Blood vessels are the body’s highways that allow blood to flow quickly and efficiently from the heart to every region of the body and back again. The size of blood vessels corresponds with the amount of blood that passes through the vessel.

All blood vessels contain a hollow area called the lumen through which blood is able to flow. Around the lumen is the wall of the vessel, which may be thin in the case of capillaries or very thick in the case of arteries. All blood vessels are lined with a thin layer of simple squamous epithelium known as the endothelium that keeps blood cells inside of the blood vessels and prevents clots from forming. The endothelium lines the entire circulatory system, all the way to the interior of the heart, where it is called the endocardium. There are three major types of blood vessels: arteries, capillaries and veins. Blood vessels are often named after either the region of the body through which they carry blood or for nearby structures. For example, the brachiocephalic artery carries blood into the brachial (arm) and cephalic (head) regions. One of its branches, the subclavian artery, runs under the clavicle; hence the name subclavian. The subclavian artery runs into the axillary region where it becomes known as the axillary artery. Arteries and Arterioles: Arteries are blood vessels that carry blood away from the heart.

Blood carried by arteries is usually highly oxygenated, having just left the lungs on its way to the body’s tissues. The pulmonary trunk and arteries of the pulmonary circulation loop provide an exception to this rule – these arteries carry deoxygenated blood from the heart to the lungs to be oxygenated. Arteries face high levels of blood pressure as they carry blood being pushed from the heart under great force. To withstand this pressure, the walls of the arteries are thicker, more elastic, and more muscular than those of other vessels. The largest arteries of the body contain a high percentage of elastic tissue that allows them to stretch and accommodate the pressure of the heart. Smaller arteries are more muscular in the structure of their walls. The smooth muscles of the arterial walls of these smaller arteries contract or expand to regulate the flow of blood through their lumen. In this way, the body controls how much blood flows to different parts of the body under varying circumstances. The regulation of blood flow also affects blood pressure, as smaller arteries give blood less area to flow through and therefore increases the pressure of the blood on arterial walls. Arterioles are narrower arteries that branch off from the ends of arteries and carry blood to capillaries.

They face much lower blood pressures than arteries due to their greater number, decreased blood volume, and distance from the direct pressure of the heart. Thus arteriole walls are much thinner than those of arteries. Arterioles, like arteries, are able to use smooth muscle to control their aperture and regulate blood flow and blood pressure. Capillaries: Capillaries are the smallest and thinnest of the blood vessels in the body and also the most common. They can be found running throughout almost every tissue of the body and border the edges of the body’s avascular tissues. Capillaries connect to arterioles on one end and venules on the other. Capillaries carry blood very close to the cells of the tissues of the body in order to exchange gases, nutrients, and waste products. The walls of capillaries consist of only a thin layer of endothelium so that there is the minimum amount of structure possible between the blood and the tissues. The endothelium acts as a filter to keep blood cells inside of the vessels while allowing liquids, dissolved gases, and other chemicals to diffuse along their concentration gradients into or out of tissues.

Precapillary sphincters are bands of smooth muscle found at the arteriole ends of capillaries. These sphincters regulate blood flow into the capillaries. Since there is a limited supply of blood, and not all tissues have the same energy and oxygen requirements, the precapillary sphincters reduce blood flow to inactive tissues and allow free flow into active tissues. Veins and Venules: Veins are the large return vessels of the body and act as the blood return counterparts of arteries. Because the arteries, arterioles, and capillaries absorb most of the force of the heart’s contractions, veins and venules are subjected to very low blood pressures. This lack of pressure allows the walls of veins to be much thinner, less elastic, and less muscular than the walls of arteries. Veins rely on gravity, inertia, and the force of skeletal muscle contractions to help push blood back to the heart. To facilitate the movement of blood, some veins contain many one-way valves that prevent blood from flowing away from the heart. As skeletal muscles in the body contract, they squeeze nearby veins and push blood through valves closer to the heart.

When the muscle relaxes, the valve traps the blood until another contraction pushes the blood closer to the heart. Venules are similar to arterioles as they are small vessels that connect capillaries, but unlike arterioles, venules connect to veins instead of arteries. Venules pick up blood from many capillaries and deposit it into larger veins for transport back to the heart. Coronary Circulation The heart has its own set of blood vessels that provide the myocardium with the oxygen and nutrients necessary to pump blood throughout the body. The left and right coronary arteries branch off from the aorta and provide blood to the left and right sides of the heart. The coronary sinus is a vein on the posterior side of the heart that returns deoxygenated blood from the myocardium to the vena cava. Hepatic Portal Circulation The veins of the stomach and intestines perform a unique function: instead of carrying blood directly back to the heart, they carry blood to the liver through the hepatic portal vein . Blood leaving the digestive organs is rich in nutrients and other chemicals absorbed from food. The liver removes toxins, stores sugars, and processes the products of digestion before they reach the other body tissues.

Blood from the liver then returns to the heart through the inferior vena cava. Blood The average human body contains about 4 to 5 liters of blood. As a liquid connective tissue, it transports many substances through the body and helps to maintain homeostasis of nutrients, wastes, and gases. Blood is made up of red blood cells, white blood cells, platelets, and liquid plasma. Red Blood Cells: Red blood cells, also known as erythrocytes, are by far the most common type of blood cell and make up about 45% of blood volume. Erythrocytes are produced inside of red bone marrow from stem cells at the astonishing rate of about 2 million cells every second. The shape of erythrocytes is biconcave—disks with a concave curve on both sides of the disk so that the center of an erythrocyte is its thinnest part. The unique shape of erythrocytes gives these cells a high surface area to volume ratio and allows them to fold to fit into thin capillaries. Immature erythrocytes have a nucleus that is ejected from the cell when it reaches maturity to provide it with its unique shape and flexibility.

The lack of a nucleus means that red blood cells contain no DNA and are not able to repair themselves once damaged. Erythrocytes transport oxygen in the blood through the red pigment hemoglobin. Hemoglobin contains iron and proteins joined to greatly increase the oxygen carrying capacity of erythrocytes. The high surface area to volume ratio of erythrocytes allows oxygen to be easily transferred into the cell in the lungs and out of the cell in the capillaries of the systemic tissues. White Blood Cells: White blood cells, also known as leukocytes, make up a very small percentage of the total number of cells in the bloodstream, but have important functions in the body’s immune system . There are two major classes of white blood cells: granular leukocytes and agranular leukocytes. Granular Leukocytes: The three types of granular leukocytes are neutrophils, eosinophils, and basophils. Each type of granular leukocyte is classified by the presence of chemical-filled vesicles in their cytoplasm that give them their function. Neutrophils contain digestive enzymes that neutralize bacteria that invade the body.

Eosinophils contain digestive enzymes specialized for digesting viruses that have been bound to by antibodies in the blood. Basophils release histamine to intensify allergic reactions and help protect the body from parasites. Agranular Leukocytes: The two major classes of agranular leukocytes are lymphocytes and monocytes. Lymphocytes include T cells and natural killer cells that fight off viral infections and B cells that produce antibodies against infections by pathogens. Monocytes develop into cells called macrophages that engulf and ingest pathogens and the dead cells from wounds or infections. Platelets : Also known as thrombocytes, platelets are small cell fragments responsible for the clotting of blood and the formation of scabs. Platelets form in the red bone marrow from large megakaryocyte cells that periodically rupture and release thousands of pieces of membrane that become the platelets. Platelets do not contain a nucleus and only survive in the body for up to a week before macrophages capture and digest them. Plasma: Plasma is the non-cellular or liquid portion of the blood that makes up about 55% of the blood’s volume. Plasma is a mixture of water, proteins, and dissolved substances.

Around 90% of plasma is made of water , although the exact percentage varies depending upon the hydration levels of the individual. The proteins within plasma include antibodies and albumins. Antibodies are part of the immune system and bind to antigens on the surface of pathogens that infect the body. Albumins help maintain the body’s osmotic balance by providing an isotonic solution for the cells of the body. Many different substances can be found dissolved in the plasma, including glucose, oxygen, carbon dioxide, electrolytes, nutrients, and cellular waste products. The plasma functions as a transportation medium for these substances as they move throughout the body. Cardiovascular System Physiology Functions of the Cardiovascular System The cardiovascular system has three major functions: transportation of materials, protection from pathogens, and regulation of the body’s homeostasis. Transportation: The cardiovascular system transports blood to almost all of the body’s tissues. The blood delivers essential nutrients and oxygen and removes wastes and carbon dioxide to be processed or removed from the body. Hormones are transported throughout the body via the blood’s liquid plasma. Protection: The cardiovascular system protects the body through its white blood cells. White blood cells clean up cellular debris and fight pathogens that have entered the body.

Platelets and red blood cells form scabs to seal wounds and prevent pathogens from entering the body and liquids from leaking out. Blood also carries antibodies that provide specific immunity to pathogens that the body has previously been exposed to or has been vaccinated against. Regulation: The cardiovascular system is instrumental in the body’s ability to maintain homeostatic control of several internal conditions. Blood vessels help maintain a stable body temperature by controlling the blood flow to the surface of the skin . Blood vessels near the skin’s surface open during times of overheating to allow hot blood to dump its heat into the body’s surroundings. In the case of hypothermia, these blood vessels constrict to keep blood flowing only to vital organs in the body’s core. Blood also helps balance the body’s pH due to the presence of bicarbonate ions, which act as a buffer solution. Finally, the albumins in blood plasma help to balance the osmotic concentration of the body’s cells by maintaining an isotonic environment. The Circulatory Pump The heart is a four-chambered “double pump,” where each side (left and right) operates as a separate pump. The left and right sides of the heart are separated by a muscular wall of tissue known as the septum of the heart.

The right side of the heart receives deoxygenated blood from the systemic veins and pumps it to the lungs for oxygenation. The left side of the heart receives oxygenated blood from the lungs and pumps it through the systemic arteries to the tissues of the body. Each heartbeat results in the simultaneous pumping of both sides of the heart, making the heart a very efficient pump. Regulation of Blood Pressure Several functions of the cardiovascular system can control blood pressure. Certain hormones along with autonomic nerve signals from the brain affect the rate and strength of heart contractions. Greater contractile force and heart rate lead to an increase in blood pressure. Blood vessels can also affect blood pressure. Vasoconstriction decreases the diameter of an artery by contracting the smooth muscle in the arterial wall. The sympathetic (fight or flight) division of the autonomic nervous system causes vasoconstriction, which leads to increases in blood pressure and decreases in blood flow in the constricted region. Vasodilation is the expansion of an artery as the smooth muscle in the arterial wall relaxes after the fight-or-flight response wears off or under the effect of certain hormones or chemicals in the blood. The volume of blood in the body also affects blood pressure.

A higher volume of blood in the body raises blood pressure by increasing the amount of blood pumped by each heartbeat. Thicker, more viscous blood from clotting disorders can also raise blood pressure. Hemostasis Hemostasis, or the clotting of blood and formation of scabs, is managed by the platelets of the blood. Platelets normally remain inactive in the blood until they reach damaged tissue or leak out of the blood vessels through a wound. Once active, platelets change into a spiny ball shape and become very sticky in order to latch on to damaged tissues. Platelets next release chemical clotting factors and begin to produce the protein fibrin to act as structure for the blood clot. Platelets also begin sticking together to form a platelet plug. The platelet plug will serve as a temporary seal to keep blood in the vessel and foreign material out of the vessel until the cells of the blood vessel can repair the damage to the vessel wall.

Prepared by Tim Taylor, Anatomy and Physiology Instructor Blood Capillaries Home > Immune and Lymphatic Systems > Lymph Node Overview > Blood Capillaries Blood Capillaries The blood capillaries are where the important functions of the circulation take place: the exchange of material between circulation and cells. Capillaries are the smallest of the body's blood vessels. They are only one cell thick, and they are the sites of the transfer of oxygen and other nutrients from the bloodstream to other tissues in the body; they also collect carbon dioxide waste materials and fluids for return to the veins. They connect the tiny muscular branches of arteries, called ... Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image 2D Interactive 3D Rotate & Zoom Change Anatomical System Change View Angle Full Blood Capillaries Description [Continued from above] . . . arterioles, with tiny veins (called venules). Ultimately, the capillary is the site of internal or cellular respiration and is responsible for the utilization of oxygen by the tissue and the transporting of carbon dioxide as waste to the veins for elimination by the lungs.

The arterial blood system branches extensively to deliver blood to over a billion capillaries in the body. The extensiveness of these branches is much more readily appreciated by knowing that the capillaries provide a total surface area of 1,000 square miles for exchanges of gases, waste, and nutrients between blood and tissue fluid. Oxygen rich blood flows from arterioles or small branches of the artery into the capillary bed and the pressure inside of the arteries is roughly fifty times that on the inside of the veins. It is this pressure difference that forces the blood into the capillary bed. Although the amount of blood flowing through a particular capillary bed is determined in part by a small circular muscle around the arteriole branches, the absence of smooth muscle and connective tissue layers permits a more rapid rate of transport between the blood and the tissue.

Question: What are the tiny blood vessels called which connect the smallest arteries and veins in the body?
Answer: Capillaries